Data stored in a memory cell may be read through a pair of differential input and output signal lines (e.g., an input and output signal line and a complementary input and output signal line) during a data reading operation. The voltage difference between the input and output line and the complementary input and output line may be very small during the reading operation. To sense the small voltage difference, a sense amplifier is frequency used.
There are two common types of sense amplifiers, voltage sense amplifiers (VSA) and current sense amplifiers (CSA). VSAs are used to sense voltage differences and CSAs are used to sense current differences. In general, a current sense amplifier is used when the load of a line connected to an input port of a sense amplifier is large. In a semiconductor memory device, when the capacity of the memory is large, the pair of input and output signal lines are typically long and the load is typically very large. Accordingly, current sense amplifiers are frequently used in connection with large semiconductor memory devices. Also, due to the fact that a current sense amplifier may have a faster sense speed than a voltage sense amplifier, current sense amplifiers are often used in semiconductor memory devices having small data storage capacitors.
Referring to FIG. 1, a circuit diagram of a conventional sense amplifier is illustrated. A current source 11 provides the same amount of current to a differential input signal line INPUT and a complimentary differential input signal line INPUTB during the activation state of an sense amplifier enable signal SAEN (e.g., logic high). At this time, when data is loaded on the differential input signal line INPUT and the complimentary differential input signal line INPUTB, a current difference is generated between INPUT and INPUTB according to the voltage difference between them. A differential current sensor 12 detects the current difference between the pair of differential input signal lines INPUT and INPUTB, converts the current difference into a voltage difference, and outputs the converted voltage difference to a differential output signal line OUTPUT and a complementary differential output signal line OUTPUTB. A current sink 14 lets some of the current from the pair of differential output signal lines OUTPUT and OUTPUTB flow to a ground port VSS during the activation state of the sense amplifier enable signal SAEN. An equalization device 13 electrically connects the pair of differential output signal lines OUTPUT and OUTPUTB and equalizes them when the sense amplifier enable signal SAEN is inactive and the current source is inactive.
The ratio of the voltage difference between the pair of differential output signal lines OUTPUT and OUTPUTB to the voltage difference between the pair of differential input signal lines INPUT and INPUTB (e.g., the degree of amplification) is called the gain. The gain is controlled by regulating the sizes of PMOS transistors P13 and P14 of the differential current sensor 12 and the sizes of NMOS diodes N11 and N12 of the current sink 14. As the gain becomes larger, the sensing speed of the current sense amplifier becomes faster. However, when the gain gets too large, the signals on the pair of differential output signal lines OUTPUT and OUTPUTB may begin to oscillate, as illustrated by FIG. 8. Such oscillations may cause the levels of OUTPUT and OUTPUTB to become switched, and therefore an incorrect data value may be produced during a reading operation.
If the gain of the current sense amplifier is large, the values of OUTPUT and OUTPUTB can oscillate when the power supply voltage VCC is high. The level of the maximum power supply voltage in which oscillation is not generated in the current sense amplifier is called the High-VCC margin. Therefore, in the current sense amplifier, the gain should be appropriately controlled by regulating the sizes of the PMOS transistors P13 and P14 and the sizes of the NMOS diodes N11 and N12 so that the High-VCC margin is high enough to prevent oscillation.
In the conventional current sense amplifier, the High-VCC margin is reduced when the gain is increased in order to make the sensing speed fast. Yet, when the gain is decreased in order to increase the High-VCC margin and reduce the likelihood of oscillations, the sensing speed decreases and the rate at which data can be read from a memory also decreases. Thus, notwithstanding the desired use of current sense amplifiers in large memory devices, there continues to be a need for improved current sense amplifiers that are less susceptible to parasitic oscillators during reading operations.